+Juan Francisco Gregorio Andrade
that's not entirely true because dozens of legitimate websites are hacked and infected every day. You can get an infection just by visiting such website without clicking anything at all. The rough malware can completely ruin your Windows beyond repair and sadly a Windows reinstall is often the only solution.
The MS Security Essentials is lightweight and not slowing down much. MS SE + periodical scans with Malwarebyte's Anti-malware is good enough to keep the PC clean.